Method for setting target braking torques

ABSTRACT

In a vehicular control system in which target braking torques are set for each of the driven wheels, the smaller target braking torque for the driven wheels is determined. Based on this smaller target braking torque, a target engine torque is calculated, and the engine torque is varied to conform to the target engine torque. Residual braking torques are calculated as differences from the target braking torques for each driven wheel and the target engine torque. These residual braking torques are realized by varying the brake pressure. Therefore the target braking torque is split into an engine torque which is equal for both driven wheels and into a braking torque which may be different for each driven wheel.

PRIOR ART

DE-A1 40 30 724 discloses a control system in which target slip valuesfor the wheels of a vehicle are determined. The target slip values canconsist of slip components which are determined by an ABS and by avehicle dynamics control. Target braking torques and thus target brakingpressures for the wheels are then determined from the target slip valuesand the actual slip values. The said target braking torques and targetbraking pressures can then be converted into valve actuation times.

SUMMARY OF THE INVENTION

In the invention, the target braking torques calculated by the ABS andthe travel dynamic controller are distributed at the wheels between atarget engine torque, which is common to the drive wheels, andindividual residual torques. Therefore, the overrun torque iscontrolled. If braking does not take place (admission pressure P_(adm)=0) "select low" control takes place automatically. When there is alarge admission pressure P_(adm) >a, the engine torque disappears, whichin turn has favourable effects on the slip control. The result in theinvention is that the engine assumes the low frequency, slow control ofthe wheels while the fast control takes place via the brake.

The output variable of the subordinate brake slip controller in thevehicle dynamics controller is the target braking torque applied to therespective wheel. In the case of the nondriven wheels this torque canonly be set independently for each wheel via the brake callipers.

In the case of the driven wheels, the engine overrun torque can, withincertain limits, be additionally used as a control variable in thecoupled-in state. In this case, the overrun torque cannot be distributedindividually to the driven wheels.

This means that a wheel slip control only with the engine within thevehicle dynamics controller is a "select low" control.

If the driver additionally applies the brake hard or an active brakingintervention takes place, the wheel slip is controlled via a commoncomponent with the engine torque, and the rest is controlledindividually with the braking torque.

The distribution of the target torque between engine and brake and itscalculation is the subject matter of this invention.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a model of the drive train which is used for thecalculation of the target engine torque;

FIG. 2 is a block circuit diagram of an exemplary embodiment of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention is based on the following considerations:

The equations for the distribution of the target braking torque arederived for a vehicle driven at one axle. In principle, the method canalso be used for vehicles with all-wheel drive.

FIG. 1 shows the model of the drive train which is used for thecalculation of the target engine torque.

The following equilibrium of torques results from a nonslipping clutch:

    M.sub.1 +M.sub.2 =i*(M.sub.eng -d/dt(W.sub.eng)*⊖.sub.tot )

where:

    i=i.sub.g *i.sub.d

    ⊖.sub.tot =⊖.sub.eng +⊖.sub.k

where:

M1, M2 are driving torques at the wheels

i_(g) is the gear transmission ratio

i_(d) is the transmission ratio of the differential

⊖_(eng) is the moment of inertia of the engine

⊖_(k) is the moment of inertia of the clutch

W_(eng) is the rpm.

In addition:

    M.sub.1 =M.sub.2 =M.sub.awheel

    d/dt(W.sub.eng)=i*d/dt(v.sub.f)/r=i*bx/r

where

V_(f) : vehicle speed

bx: vehicle deceleration

r: radius of the wheel

thus the following relation applies to the overrun torque or the drivingtorque M_(awheel) at the wheel:

    M.sub.awheel =0.5*i*(M.sub.eng -i*⊖.sub.tot *bx/r)

The wheel slip controller has calculated the target torques M_(set1) andM_(set2) for the driven axle on the basis of target slip deviations.

A target torque to be realized by the engine is calculated from thesmaller of the two torques.

    M.sub.setmin =min (M.sub.set1, M.sub.set2)

according to the following formula:

    M.sub.engset =k*M.sub.setmin

where

k=(a P_(adm))/a 0<k≦1

P_(adm) : admission pressure of the driver

a: selectable parameter

Thus, the engine torque obtained is:

    M.sub.eng =2*M.sub.engset /i+i*⊖.sub.tot *bx/r

The remaining residual torques are realized via the target pressuresP_(set1) and P_(set2) in the wheel brake cylinders.

    P.sub.set1 =(M.sub.set1 -M.sub.engset)/CP.sub.1

    P.sub.set2 =(M.sub.set2 -M.sub.engset1)/CP.sub.2

where CP_(1/2) =braking torque transmission ratios.

In FIG. 2 a block circuit diagram of an exemplary embodiment of theinvention is shown. At terminals 2, target brake slip values λ_(s1) andλ_(s2) and actual brake slip values λ_(i1) and λ_(i2) of the two drivenwheels (1 and 2) which are determined in a known travel dynamiccontroller and an ABS are fed to a controller 3. The latter determinesthe target braking torques M_(set1) and M_(set2) of the two wheels. In ablock 4, the smaller torque is selected. In a further block 5, theexpression 2k/i)M_(setmin) is formed, to which the summand i-bx/racquired in block 7 is added in block 6. The sum influences the enginetorque (block 8) and must be realized by the engine by means of theappropriate setting of the throttle valve.

In blocks 9a and 9b the differences M_(R1) =M_(set1) -kM_(setmin) andM_(R2) =M_(set2) -kM_(setmin) are formed and are converted in blocks 10aand 10b into braking pressures P₁ and P₂. These signals are fed via aninverse hydraulic model 11a and 11b which determine valve operatingtimes, to brake pressure control valves 12a and 12b, by means of whichthe residual torques are applied to the wheels. Such an inversehydraulic model is described in DE-A1-40 30 724.

We claim:
 1. Method for controlling slippage of driven wheels in a motorvehicle, said method comprisingdetermining target braking torquesM_(set1) and M_(set2) for respective driven wheels, determining asmallest torque M_(setmin) of the two torques M_(set1) and M_(set2),calculating a target engine torque M_(engset) from the smallest torqueM_(setmin), varying engine torque M_(eng) to conform to M_(engset),calculating residual braking torque M_(R1) based on M_(set1) andM_(engset) and residual braking torque M_(R2) based on M_(set2) andM_(engset), and varying brake pressure at the driven wheels so that theresidual torques M_(R1) and M_(R2) are realized.
 2. Method as in claim 1wherein said engine torque is calculated according to

    M.sub.eng =2*kM.sub.setmin /i+i*⊖.sub.tot *bx/r

where i is the combined transmission ratio of gear box and differential,⊖_(tot) is the moment of inertia of the engine and gearbox, bx is thevehicle deceleration, r is the wheel radius, and k is a variable between0 and 1 which becomes small with increasing brake pressure P_(adm)applied by the driver.
 3. Method as in claim 1 wherein

    M.sub.R1 =M.sub.set1 -M.sub.setmin and M.sub.R2 =M.sub.set2 -M.sub.setmin.


4. Method as in claim 3 wherein brake pressure is varied at the drivenwheels by converting M_(R1) and M_(R2) to brake valve operating times.